This invention pertains generally to methods and formulations for stopping or severely restricting a catalytic reaction between hydrogen and oxygen within enclosed spaces wherein the mixture of hydrogen and oxygen is within an explosive range. In particular, the invention pertains to a material formulation comprising a hydrogen getter and an inert low melting point agent which when heated will melt and wet the surface of the getter and thereby terminate the catalytic reaction before that reaction reaches the auto-ignition temperature of hydrogen.
Hydrogen can be produced from corrosion of metals by atmospheric gases or electric currents; from batteries, and sealed electronic devices operating in normal or abnormal conditions. In particular, the generation and accumulation of hydrogen in these devices can, and frequently does, become a safety hazard. For instance portable lighting devices such as sealed lanterns and flashlights which employ batteries having aqueous electrolytes, must provide a means for safely eliminating hydrogen generated as the result of electrochemical reactions, charging, excessive discharge, or inadvertent battery reversal, etc.
Of particular concern, is the presence of both hydrogen and oxygen because of the potential for an explosion due to a static electric discharge or a spark in the presence of a noble metal hydrogenation catalyst. It will be appreciated by those skilled in the art, that the reaction between hydrogen and oxygen in the presence of a hydrogenation catalyst is rapid and exothermic and over a wide range of concentrations can be explosive because the catalyst temperature can rise from the heat of reaction above the ignition temperature of the mixture of hydrogen and oxygen gases.
It has long been known that hydrogen scavenging materials, known as xe2x80x9cgetters,xe2x80x9d can be used to counteract hydrogen accumulation. Ayers et al. (U.S. Pat. No. 4,512,721) discusses the use of active metals such as zirconium or titanium, and alloys thereof. These metals are capable of maintaining low hydrogen partial pressures but have the disadvantage of requiring high temperatures for initial activation and/or ongoing operation because of the necessity to diffuse surface contaminants into the bulk metal thereby providing a fresh surface for continued hydrogen absorption.
It is also well known in the art that unsaturated carbon-carbon bonds (i.e., double or triple bonds between carbon atoms) can be reduced by hydrogen and its isotopes in the presence of an appropriate catalyst to form an alkane; see, for example, Fieser, L. F. and Fieser, M., Textbook of Organic Chemistry, D. C. Heath and Co. 1950, pp. 66-69 and 86. Additionally, Anderson et al., (U.S. Pat. Nos. 3,896,042 and 3,963,826) and Harrah et al., (U.S. Pat. No. 4,405,487) disclose the use of solid acetylenic compounds (i.e., organic compounds having carbon-carbon triple bonds) combined with various Group VIII metal catalysts to irreversibly remove hydrogen over the temperature range xe2x88x9250xc2x0 C. to 110xc2x0 C. Shepodd et al. (U.S. Pat. Nos. 5,624,598, 5,703,378, 5,837,158 and 6,063,307) disclose other compositions of unsaturated organic compounds and noble metal catalysts, particularly Pd, suitable for gettering hydrogen under a variety of conditions.
Finally, Labaton, (U.S. Pat. No. 4,886,048), describes another means for removing hydrogen by reacting the hydrogen with oxygen in the presence of a noble metal catalyst, such as palladium, to form water and then trapping the water on a water absorbing material such as a molecular sieve; and Shepodd, et al. (U.S. Pat. No. 5,998,325), disclose a hydrogen getter composition suitable for use in gaseous mixtures of hydrogen and oxygen within the explosive range, wherein the hydrogen and oxygen are catalyzed to form water vapor.
Under most normal operating conditions hydrogen getters of the type described by Shepodd, above (""325), operate effectively and without need for emergency intervention. However, in order to protect a user in situations where the hydrogen getter becomes dangerously overheated there is a need for a means to safely remove hydrogen from an atmosphere which contains oxygen without the danger of explosion. The present invention provides a means for restricting the catalyzed reaction between hydrogen and oxygen from proceeding so rapidly as to accelerate out of control: to begin to xe2x80x9crun-away.xe2x80x9d
Certain consumer products such as sealed lanterns, flash lights, and the like, which rely on alkaline cells to provide power, can produce and retain hydrogen gas when the device is used and the electrical cells are discharged. Since these devices must also necessarily include an electrical switch, which may act as a spark source, these devices have been designed to incorporate a hydrogen getter pellet to safely remove the hydrogen gas. Unfortunately, such designs require making certain assumptions regarding the level of probable hydrogen generation which sometimes dictate increasing the concentration of these catalysts in order to increase the reaction rates of the hydrogen when the ambient concentrations of hydrogen are low. However, in certain circumstances there may exist a high probability that the getter pellet will be exposed to large quantities of hydrogen, such as during a cell short-circuit or reversal. In these cases such formulae can become increasing hot in the presence of higher concentrations of hydrogen and can thus present a xe2x80x9crun-awayxe2x80x9d hazard; that is, a situation in which the catalysis reaction begins to accelerate uncontrollably and causing the surface temperature of the getter to increase so quickly such that the system is unable to disperse the heat of reaction will lead to the potential for an ignition to occur. Such scenarios can and have lead to catalyst surface temperatures exceeding the auto-ignition temperature of hydrogen when a sufficient concentration of oxygen is also present. Such devices, therefore, are deemed to pose an imminent hazard if oxygen is also present in any significant quantity.
Given that there is a desire on the part of a manufacturer to provide a product which can be safely operated under even the most extreme abuse, for those products or devices which incorporate a hydrogen getter to consume spurious hydrogen gas generated during device operation, it would be desirable to provide a means for moderating that reaction so as to prevent the getter from generating excessive amounts of heat and getting hot. This is especially desirable in instances where the atmosphere into which the hydrogen evolves also contains a significant fraction of oxygen.
It is therefore an object of this invention to provide a getter composition which includes a heat-activated sacrificial material intended to greatly slow gettering above a chosen temperature. In particular, it is an object of this invention to provide a wax, or similar material, for compounding with a getter powder which would melt, wet, and coat the getter powder at a predetermined temperature and thereby greatly reduce the effective getter surface available for reaction.